What is a polar aurora?
A polar aurora is produced by the collision of an ejection of solar mass with the north and south poles of the earth’s magnetosphere. This collision causes a diffuse but predominant glow projected on the earth’s atmosphere.
The polar auroras emerge as two centered ovals extending around the earth’s magnetic poles, which are near but do not coincide with the geographic poles.
This phenomenon takes place when charged particles (protons and electrons) coming from the sun, guided by the magnetic fields of the earth, have an impact on the atmosphere near the poles. When these particles collide with the atoms and the oxygen and nitrogen molecules, which are the predominant components of the air, part of the energy of the collision disturbs those atoms and molecules. As a result of the friction, very high levels of energy are produced. Shortly afterwards, on the order of a millionth of a second, the atoms and the molecules recede to their original level and the energy is converted into light again. That same light is the one we see from the ground and so- call “aurora.”
The sun, which is 150 million kilometers away from the earth, is constantly sending out charged particles – positively charged protons and negatively charged electrons. That ongoing flux of particles is called solar wind. The charged particles that are found in the sun’s atmosphere tend to move away from the sun. Once they are accelerated, they move along the magnetic field of the sun reaching the earth’s orbit and far beyond.
The solar wind particles away from the sun at speeds ranging from 300 to 1000 km/s, covering the sun-earth distance in, approximately, two days. In the vicinity of the earth, the solar wind is diverted by the earth’s magnetic field or magnetosphere. The particles flow into the magnetosphere in the same way a river flows around a rock, or the pier of a bridge. The solar wind pushes the magnetosphere and distorts its. Instead of exhibiting a uniform beam emitted by a magnetic field like the one that an imaginary magnet placed in the inside of the earth with a north-south direction would display, it exhibits a lengthened structure in the shape of a comet with a long tail moving away from the sun. The charged particles end up being the first to get stuck and to travel along the lines of the magnetic field and follow the signaled course.
The polar auroras occur between 95 and 100 kilometers above the earth's surface. They stay above 95 km because at that altitude the atmosphere is so dense, and the collisions with charged particles happen so frequently, that the atoms and molecules are practically at rest. On the other hand, the auroras cannot be above 500 and 1,000 km because the atmosphere is too faint for the collisions to show any significant effects.
Auroras have different names depending on the places where they can be seen. An aurora is called borealis when the phenomenon is observed in the northern hemisphere and it is called australis when it is observed in the southern hemisphere. But there is no other difference between them.
Colors and shapes of auroras
Auroras have different shapes and very diverse colors. Besides, they change very quickly over time.
At night, an aurora can start forming a very long arc extending from the horizon, generally in the east-west direction. Near midnight, the arc may become brighter. Waves and curls start shaping along the arc. Auroras also exhibit vertical structures that look like very thin and long light beams. They may extend over the whole sky (from horizon to horizon) for hours or for a few minutes. When daybreak is near, the process seems to recede and only small parts of sky glow until morning comes.
The different colors we see in auroras are determined by the type of atoms or molecules that the wind particles of solar wind excite, and of the level of energy reached by those atoms and molecules.
Oxygen is responsible for the two primary colors in auroras – green and yellow. A red aurora, occasionally seen, is produced by a different process.
A collision can rip external electrons from nitrogen. This process can create a blue light, whereas red light is created by nitrogen molecules and it appears on the lower fringes of auroras and in the most eternal curved sections.
Auroras on other planets
This phenomenon is not privative of the earth. Other planets in the solar system exhibit similar phenomena, like the case of Jupiter and Saturn. These planets´ magnetic fields are stronger than the earth’s (Uranus, Mercury, and Saturn also have magnetic fields) and both have broad radiation belts. Auroras in both planets have been observed with the Hubble telescope.
These auroras seem to be powered by solar winds. In addition, Jupiter's moons, especially Io, are also powerful sources of auroras. These arise from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to relative motion between the rotating planet and its moving moons. Io, which has active volcanism and an ionosphere, is a particularly strong source, and its currents also generate radio emissions that have been studied since 1955.
In Mars, auroras have also been detected by the ship Mars Express during an expedition that took place in 2004. The conclusions were published a year later. Mars lacks a magnetic field with the same characteristics than the earth’s, but it has local magnetic fields associated with its crust. Seemingly, these are responsible for the auroras in this planet.
When do polar auroras take place? When is it possible to observe this phenomenon?
Because the particles of solar wind are continuously reaching the earth there are always auroras both during the day and during the night. Obviously, its auroras cannot be seen during day time since the light of the sun is much more intense.
The following factors make observation more likely –
- Time of day: Because the intensity of an aurora’s glow is very low, it can only be observed at night. In fact, the most active and bright ones come into sight at midnight. So the best time to observe them is between 11.00 p.m. and 2.00 a.m.
- Season: At the latitudes where auroras are more frequent, in summer, there is daylight practically during the whole day. Autumn and spring are ideal due to the fact that the temperature is not too low and that night is longer. In most polar areas, weather tends to be good and days tend to be bright in the middle of winter. Observations can also be conducted in that season.
- The solar activity cycle: Every 11 years the sun has a record of magnetic activity. That is why the more frequent the solar activity is the more frequent auroras are. It is also possible that the range of latitudes where this phenomenon can be observed extends further north in the southern hemisphere and vice versa. However, it should be pointed out that bright and intense auroras are observed any time in the solar cycle.
- Sun’s rotation: When one polar aurora takes place, another is very likely to appear after 27 days. This happens because the sun takes 27 days to rotate on its axis. The aurora taking place after that period will be weaker than the first.
- Moon phase: Observing auroras when the moon is full or when this moon phase is near should be avoided. Looking at them if the moon is very high on the horizon should also be avoided.
- Location: Further north or further south, there will be a greater probability to see a polar aurora. However, it is important to consider that the positions close to southern and northern magnetic poles are not suitable. Greenland, the north of Canada, and Alaska are the privileged places in the north and, in the south, Antarctica is perfect. The same happens in the south of Australia and New Zealand, where the phenomenon can be seen quite frequently.